1. Field of the Invention
The invention relates to a device for torque transmission, and is concerned more particularly with a device of this kind which includes a fluid coupling device, which is generally interposed in an automotive vehicle between a driving shaft such as the engine crankshaft and a driven shaft such as the input shaft of an automatic gearbox, so as to perform the function of a clutch.
2. Description of the Prior Art
A torque transmitting device of this kind is known, which includes, housed in a common casing and rotating in parallel, a hydraulic coupling member or torque converter together with a mechanical clutch of the kind known as a locking clutch.
Such a torque transmitting device is for example described in British Patent Application No. GB-A-2 045 367.
In this construction, the clutch is, on starting and generally at each change of transmission ratio, ineffective during an initial period of time. It is therefore the hydraulic coupling that is initially left to act between the input shaft or crankshaft and the output shaft (the input shaft of the gearbox). During a second period of time, the clutch causes the hydraulic coupling to be taken out of service so as to eliminate any slip in the hydraulic coupling, thus improving the performance of the assembly by ensuring direct mechanical coupling between the two shafts.
During this second stage, the hydraulic system is thus bypassed.
In other forms of the device, the hydraulic coupling is arranged in the rotating casing, while the clutch is situated outside the casing.
In this case, two concentric driven shafts are provided, and transmission of the torque is obtained alternatively through one or the other of these driven shafts, so that the torque is transmitted either through the hydraulic system or through the mechanical system.
More precisely, one of the driven shafts is connected in rotation to the casing and is surrounded by a second driven shaft, which is connected in rotation to a turbine wheel of the hydraulic coupling.
In the remainder of this document, such an assembly will for convenience be referred to as a "fluid coupling device", which may consist of a torque conversion apparatus or of a simple coupling device. Activation of the clutch, that is to say of the mechanical device for locking the two shafts together, is obtained through hydraulic control for any chosen speed. In general, it is desirable to choose a speed at which this locking is to take place which is quite small, for energy conservation reasons; however, in achieving this there is a risk of encountering all the conventional vibration problems, which are mainly due to torsional oscillations originating from the engine. A principal object of the invention is to overcome this disadvantage, and to provide a torque transmission device of the fluid type, in which mechanical locking together of the two shafts takes place at a low speed.